1. Field of the Invention
This invention generally relates to electrostatic recording, and particularly to a method of electrostatically transferring to a recording medium, in a single operation, a toner image consisting of toner particles having different polarities.
2. Description of the Prior Art
At the present time, various types of recording apparatus are required in the field of business machines and computers. In such recording apparatus, contents to be recorded may be classified into fixed-type information having fixed content such as forms, and variable-type information as may be presented by different characters, numerals and the like. An example of distinguishing between fixed-type information and variable-type information is to employ colors. Many attempts have been proposed for recording colors according to the type of contents. Of course, colors can be used not only to distinguish the mentioned types of contents but also to distinguish other information. In such electrostatic recording methods as employ electrophotography, or multiple styli, or an electrode having a pattern contour, an image is developed on a latent image retentive medium with two kinds of toner particles having different polarities and colors, thereby providing two kinds of recording information in a discriminatable manner. Illustratively, such electrostatic recording may involve formation of a latent image having different polarities on an electrostatic sheet serving as a latent image retentive medium, this latent image being then developed with toner particles having different polarities and colors. With this illustrative prior art method, a satisfactory two-color toner image may be obtained on a latent image retentive medium.
However, the indicated prior art lacks ability to simultanteously transfer an image consisting of two different kinds of toners having positive and negative polarities, from the electrostatic latent image retentive medium, onto a recording medium such as ordinary paper. For this reason, it has been a common practice in recording an image to fix a toner image on a costly latent image retentive medium, and then to use the medium as the recording sheet.
Xerography and the PIP method are among prior art methods for two-color recording. According to these methods, a latent electrostatic image having one polarity, either positive or negative, is first subjected to development of one color toner, transfer to a recording medium, and fixing; and thereafter a latent image of another polarity is subjected to a similar procedure, using another color toner. Such two-color recording, however, dictates an expenditure of time twice as long as that required for a method for one color recording and hence is not adaptable to high-speed recording.
A transfer method is also part of the prior art for two-color recording. In this method, a latent electrostatic image having two polarities is subjected to development by two color toners, and a recording medium is superposed on a toner image having two polarities. Then, a conductive roller is contacted to the back surface of the recording medium, and A.C. voltage is applied between the conductive roller and a latent image retentive medium on which a toner image is positioned. However, such a method is low in efficiency and the transferred toner image on the recording medium is uneven, because when the polarity of A.C. voltage becomes the same polarity as the toners of the toner image, these toners cannot be transferred to the recording medium.